Conversion Reactions of Various Phenoxyalkanoic Acid Herbicides in Soil. 1. Enantiomerization and Enantioselective Degradation of the Chiral 2-Phenoxypropionic Acid Herbicides

The degradation of selected phenoxyalkanoic acid herbicides in soil under laboratory conditions was studied by using enantioselective high-resolution gas chroma tography/mass spectrometry (HRGC/MS). The compounds investigated were the chiral 2-(4-chloro-2-methylphenoxy)propionic acid (MCPP) and 2-(2,4-dichlorophenoxy)propionic acid (DCPP) and the achiral (4-chloro-2-methylphenoxy)acetic acid (MCPA), 2,4-(dichlorophenoxy)acetic acid (2,4-D), and dicamba, a benzoic acid derivative. Racemic and enantiopure MCPP and DCPP were incubated in separate experiments. In case of MCPP and DCPP, the herbicidally active R enantiomers were significantly slower degraded than the inactive S enantiomers. Incubation of enantiopure MCPP and DCPP revealed significant enantiomerization with formation of the R enantiomers from the S enantiomers, and vice-versa. Enantiomerization was found to be biologi cally mediated and was not observed in sterilized soil. Degradation followed initially approximate first-order kinetics with ove...